I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for automatic gain control in a communications receiver.
II. Description of the Related Art
Wireless devices utilize radio waves to provide long distance communications without the physical constraints of a wire-based system. Information is provided to devices using radio waves transmitted over predetermined frequency bands. Allocation of available frequency spectrum is regulated to enable numerous users access to communications without undue interference.
A remote receiver tuned to a carrier frequency is required to receive and demodulate signals transmitted from a corresponding transmitter at the same carrier frequency. The remote receiver recovers the baseband signal from the modulated carrier. The baseband signal may be directly presented to a user or may be further processed prior to being presented to the user.
A mobile receiver in a portable communication system operates in an environment that subjects the receive signal to numerous degradations. The signal transmitted from a signal source is subject to numerous conditions, such as attenuation, interference, scattering, and reflections, prior to arrival at a receiver. The receiver must be able to recover the signal in spite of all these degradations in order for a successful communication link to be established.
Structures, such as buildings, and surrounding terrain, including walls and hillsides, contribute to the scattering and reflection of the transmitted signal. The scattering and reflection of the transmit signal results in multiple signal paths from the transmitter to the receiver. The contributors to the multiple signal paths change as the receiver moves.
Other signal sources also result in degradation of the desired signal. The other signal sources may be other transmitters intentionally operating on the same frequency as the desired signal as well as transmitters that generate spurious signals in the frequency band of the desired signal.
Other sources of signal degradation are generated within the receiver itself. Signal amplifiers and signal processing stages within the receiver may degrade the level of the desired signal with respect to the level of thermal noise. The signal amplifiers and processors within the receiver may also generate noise products or distort the received signal and further degrade its quality.
Receivers designed to operate within defined communication systems must be designed to operate in spite of all sources of signal degradation. In an exemplary embodiment, the wireless communication system may be a system such as a Code Division Multiple Access (CDMA) wireless system, consistent with “Telecommunications Industry Association (TIA)/Electronics Industries Association (EIA)/IS-2000 STANDARDS FOR CDMA2000 SPREAD SPECTRUM SYSTEMS” referred to as “the cdma2000 standard.” In alternate embodiments, the system may be a system consistent with the “TIA/EIA/IS-95 MOBILE STATION-BASE STATION COMPATIBILITY STANDARD FOR DUAL-MODE WIDEBAND SPREAD SPECTRUM CELLULAR SYSTEM,” hereinafter referred to as “the IS-95 standard,” or other systems such as described by American National Standards Institute (ANSI) “J-STD-008, PERSONAL STATION-BASE STATION COMPATIBILITY REQUIREMENTS FOR 1.8 TO 2.0 GHZ CODE DIVISION MULTIPLE ACCESS (CDMA) PERSONAL COMMUNICATIONS SYSTEMS,” or “ANSI J-STD-015 DRAFT STANDARD FOR W-CDMA (WIDEBAND CODE DIVISION MULTIPLE ACCESS) AIR INTERFACE COMPATIBILITY STANDARD FOR 1.85 TO 1.99 GHz PCS APPLICATIONS” referred to as “W-CDMA,” or other systems generally referred to as High Data Rate (HDR) systems.
In addition to operating in a noisy environment, a receiver must be capable of handling input signals varying over a large range. A typical receiver operating in a system defined by one of the standards listed above must be capable of handling an input signal range of 100 dB or more. However, a typical signal amplifier remains linear over a much smaller range. A receiver typically incorporates some form of gain control in order to maintain signal linearity at the later stages of the receiver. Gain control may take the form of switched amplifiers, variable gain amplifiers, or a combination of switched and variable gain amplifiers. Additionally, since a receiver may utilize both analog and digital stages, gain control may control the gain of analog stages of the receiver prior to an Analog to Digital Converter (ADC) and may also control the gain of the signal in digital stages following the ADC.
The typical gain control circuit performs Automatic Gain Control (AGC), although it is possible to implement manual gain control in receivers having slowly varying signal levels. The AGC may be implemented using discrete gain steps, constant variable gain, or a combination of the two. An AGC circuit, in a receiver having analog and digital stages, is typically implemented in the analog portion of the receiver in order to optimize the signal level input to the ADC. The AGC is configured such that the mean energy level of the received signal at the input to the ADC is maintained at a predetermined level. This predetermined signal level, termed the AGC set point, is typically chosen to be a number that results in ADC input signal levels near the middle of the ADC range. The design of the digital stages can be optimized to the AGC set point value. The bitwidths of subsequent digital stages may be optimized to the AGC set point. A higher AGC set point may require a larger bitwidth in the subsequent digital stages to ensure the signal does not saturate the digital processing stage. A lower AGC set point, correspondingly, may require a smaller bitwidth in subsequent digital stages. The actual bitwidths implemented in the digital portion of the receiver design are of greater interest when stages of the digital portion of the receiver are implemented within a single Integrated Circuit (IC). A larger bitwidth typically will require a larger physical area and the consumption of more resources within a single IC. Thus, it may be preferable to limit the bitwidths within an IC design to no greater than a minimum level.
The bitwidths of the subsequent digital stages will typically be designed to accommodate the worst case AGC set point when the AGC set point is allowed to vary over the whole range of the ADC. This results in an excessive bitwidth and the consumption of a limited amount of resources within the IC when a lower AGC set point is used in the receiver design.
A similar situation exists for receivers that implement analog stages following the AGC stages. In such a receiver the AGC set point is determined prior to subsequent analog stages. The subsequent analog stages may have limited dynamic ranges, as in the case of an amplifier. It is preferable to design the subsequent analog stages such that an amplifier is linear over the dynamic range determined by the AGC set point. It may not be desirable to implement an amplifier having a dynamic range far exceeding that determined by the AGC set point since both physical space and excessive power is consumed. However, when the AGC set point is allowed to vary over a predetermined range, the subsequent analog stages will typically be designed to accommodate the worst case AGC set point.
In both the digital and analog designs the subsequent stages are typically designed to accommodate the worst case4 AGC set point. If the actual AGC set point of the receiver is not designed to be the worst case AGC set point the result is a non-optimal design for the subsequent stages and the use of excessive resources. What is needed is a method and apparatus for making the receiver design insensitive to the AGC set point value.